Main.swift

protocol OptionalType {
    typealias T
    func flatMap<U>(@noescape f: (T) -> U?) -> U?
}

extension Optional : OptionalType { }

extension SequenceType where Generator.Element: OptionalType {
    func flatten() -> [Generator.Element.T] {
        return self.map { $0.flatMap { $0 } }
                   .filter { $0 != nil }
                   .map { $0! }
    }
}

let z: [Int?] = [1, 2, nil, 3]

let flatz = z.flatten()     // 1, 2, 3